WASHINGTON (CNN) -- Fossilized bones of a saber-toothed cat and dinosaurs that may be 100 million years old are among "priceless" artifacts that the United States handed over to China in a ceremony Monday.

This skull of a saber-toothed cat was among the fossils seized by federal agents and returned to China.

The fossils, which also include dinosaur eggs believed to be 60 million years old, were seized by U.S. authorities when people tried to bring them into the United States through mail or luggage, U.S. Immigration and Customs Enforcement told CNN.

"These pre-historic fossils are an invaluable part of the history of the People's Republic of China and they will undoubtedly contribute to the scientific exploration of that nation's past," said John Morton, the assistant secretary of Homeland Security who oversees ICE, in a statement before the ceremony at the Chinese Embassy.

Xie Feng, deputy chief of mission at the embassy, expressed "heartfelt gratitude to the U.S. government for its efforts to return the fossils to China," the statement said. Watch how ancient bones were found »

The head of the saber-toothed cat has an open jaw with long, sharp teeth. It measures eight inches high, about 12 inches from the teeth to the back of the head, and just over eight inches in width, said ICE spokeswoman Pat Reilly.

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The head of one of the dinosaurs -- a Psittacosaurus lujiatunesis, which were small -- is just under seven inches high, seven inches front to back, and seven inches ear to ear, said Reilly.

These were among the items that entered the United States through the mail.

In December 2006 and October 2007, Customs and Border Patrol agents inspecting shipments through the Chicago O'Hare International Mail Facility found three parcels containing the collection of fossils. When X-ray images showed that the packages did not seem to match declared contents, the parcels were passed on to ICE for investigation.

"The attempt to remove them from China ran up against a network of national and international customs laws that are in place to protect against the theft of cultural property. We are pleased to return them to their rightful owners, the people of China," Morton said in the statement.

"The fossils in Chicago were evaluated by experts at the Field Museum, who determined them to be animal remains dating back as far as 100 million years," ICE said in the statement.

ICE spokeswoman Reilly said there is "a big black market" for such items. Though "priceless" and "invaluable" from a historic perspective, the cat and dinosaur fossils and some other fragments are valued at about $30,000 combined, she told CNN.

The collection of 24 fossilized eggs from several dinosaurs was brought in by a passenger at Washington Dulles International Airport.

Customs agents found them and had them shipped separately to Richmond, Virginia, where the passenger was ending his journey. Experts from the Virginia Museum of Natural History authenticated the eggs and determined them to be about 60 million years old, ICE said in the statement.

The eggs are worth nearly $40,000, Reilly said.

No criminal charges were filed, she said. But those who brought in the artifacts lost every penny they may have spent.

Countries have special documents permitting people to transport such artifacts when it is allowed, Reilly said.

Those who brought the items into the United States did not necessarily get them in China. "What often happens is that the person who is actually transporting the thing acquired it in a third country," she said.

In recent years, several countries have been stepping up efforts to return precious historic artifacts to China.

Xie Feng of the Chinese Embassy cast Monday's development as part of an era of increased U.S.-Chinese cooperation.

"In recent years, China and the U.S. have developed close cooperation in law enforcement and made steady progress and prominent achievements, particularly in the fields as counter-terrorism, drug enforcement as well as combating other transnational crimes," he said in the statement.

"Such law enforcement cooperation will benefit the safety of our countries and the protection of our people's lives and property."